This invention relates to the measuring of a characteristic magnitude of the operation of a timepiece movement and, more particularly, to a device for measuring said magnitude by detecting the mechanical vibrations produced in the operation of said movement.
The characteristic magnitudes of the operation of a timepiece movement are for instance the time error, the dissymmetry of the half-oscillations of the balance or the amplitude of oscillation of the balance. For measuring those magnitudes the vibration produced by the operation of the timepiece movement, in particular, in the case of a balance-wheel watch, the vibrations produced by the operation of the escapement, are detected and converted into electric pulses and those measuring pulses are compared with a periodic reference signal or clock pulse of a known constant frequency. A device of this type is for instance described in Swiss Pat. No. 556 573 which corresponds to U.S. Pat. No. 3,817,083.
Measuring errors can be produced in such a device by undesired vibrations or noise signals which generally occur at irregular intervals between the periodic pulses of the operation of the timepiece. Those interfering pulses can produce measuring pulses which are not related to the characteristic magnitude of the timepiece to be measured.
It is already known to prevent the generation of measuring pulses by interfering noise pulses, by blocking the generation of measuring pulses during a part of the time interval between two consecutive normal pulses produced by the timepiece movement, which blocking interval starts immediately after each measuring pulse and ends soon enough to allow the generation of a measuring pulse by the following vibration due to the normal operation of the watch movement.
In other words, each time interval between two consecutive useful pulses produced by the watch movement comprises a blocking period during which the generation of measuring pulses is prevented, and a window during which vibrations detected by the device will generate a measuring pulse. To reduce in as far as possible the generation of a measuring pulse by an interfering pulse, the duration of the blocking period should be as close as possible to the duration of the time interval between two consecutive useful measuring pulses, and therefore the said window should be open only at the expected time of arrival of a useful measuring pulse.
However, prior to the invention, it has not been possible to increase the ratio between the blocking period and the time interval between two consecutive useful measuring pulses above a relatively low limit and, consequently, to reduce the window below a relatively large part of the said interval. This is due to the fact that the said time interval between two consecutive useful measuring pulses is not known very precisely, at least before it has been measured, and therefore the duration of the window must be sufficiently long to make sure that the expected measuring pulse will fall within said window. In particular, when measuring the half-period of the oscillation of a balance-wheel or other dissymmetric time intervals, the blocking period must be limited as far as necessary for measuring the shorter time interval. For instance, for a watch movement with an oscillation frequency corresponding to 36,000 half-periods per hour or a period of a duration of 200 milliseconds, the shortest time interval between two consecutive useful measuring pulses may be 90 milliseconds and the longest time interval can be 110 milliseconds. In this case the maximum value of the blocking period is in practice limited to 80 milliseconds, which means that the window is 10 milliseconds for the short interval and 30 milliseconds for the long interval. For an oscillation frequency corresponding to 18,000 half periods per hour or a period of 400 milliseconds, practical values are for instance the following: Duration of the short interval 190 milliseconds, duration of the long interval 210 milliseconds, blocking period 80 milliseconds, duration of the window in the short interval 110 milliseconds, duration of the window in the long interval 130 milliseconds.
Therefore, the risk of a generation of a measuring pulse by interfering noise remains important in spite of the presence of a blocking period.